US1833806A - Electrolytic refining of aluminum - Google Patents

Description

Nov. 24, 1931. .1. WEBER ET AL ELECTROLYTIC REFINING OF ALUMINUM Filed Jan. 8, 1929 CaT/mde a Patented Nov. 24, 1931 UNITED STATES PATENT OFFICE JULIUS WEBER AND ALFRED V011 ZEEIBLEDER, OI NEUHAUSEN, SWITZERLAND, A3-

SIGNORS TO ALUMINIUM INDUSTRIE AKTIENGESELLSCEAF'I', OI NEUHAUSEN,

SWITZERLAND, A JOINT-STOCK COMI'ANY OF SWITZERLAND ELECTROLYTIC REFINING OI ALUMINUM Application filed January 8, 1929, Serial No.

This invention relates to the electrolytic refining of aluminum, and includes a novel process for producing highly pure metallic aluminum from less pure metal, and a spe- .6 cial electrode used therein. The invention is directed particularly to the extraction of refined aluminum in a solid and dense form from pig or ingot aluminum, aluminum alloys and the like. It constitutes an improvement more particularly in that refining processes wherein the electrodes, used in a solid form, are immersed in a molten electrolyte, for example containing halogen salts, which electrolyte has a melting point lower than that of either electrode and is maintained in a molten condition below the melting point of either electrode. Such a process has been described in and made the subject of the prior application of Weber & Hauser, Serial N 0. 113,561, filed June 3, 1926, patented April 16, 1929, No. 1,709,759, also in an improved form in the application of Weber & Zeerleder, Serial No. 195,861, filed June 1, 1927. These prior applications may be referred to for details and features not herein fully described.

The general objects ofthe present invention are to render more efiicient the refining process, to increase the purity of the product,

30 and to enlarge the rate of output or production. Other objects and advantages of the invention will be explained in the hereinafter following description of an illustrative embodiment thereof or will be understood to those conversant with the subject. To the attainment of such objects and advantages the present invention consists in the novel electrolytic process of producing pure metallic aluminum, and the novel materials or electrodes used therein, and the novel features of process, operation and means herein described or illustrated.

The nature of the present improvement can best be explained by first pointing out the conditions which pertain to the processes already known and the considerations involved in further perfecting or im roving the same. In practicing thedescribe refining process we have determined that the aluminum which is deposited in solid form at the cathode may solved to a depth of several millimeters,

380,987, and in Germany February 4, 1928.

be rendered the more pure by maintaining the current density lower at the anode. The reason underlying this is that the selective action of the electric current on the various constituents of the anode is far more effective at low than at high current densities. By the use of low current densities it is possible particularly to restrain the taking up of iron at the anode, although iron, which is invariably present in pig aluminum, has a considerably greater tendency to enter into solution than other constituents, for example silicon, which is an equally unavoidable and undesirable impurity in pig aluminum.

While, as stated, the use of lower current density at the anodes is found to enhance the purity, on the other hand We find that far igher current densities are permissible at the cathodes without impairing the purity of the deposited metal.

Observation of the described process shows that the electrolytic tension of the bath commences to increase at a rapid rate as soon as the aluminum at the anode has been disthe remaining undissolved constituents, consisting chiefly of iron and silicon, forming a sort of coating or crust. This coating of impurities upon the anode acts as a membrane or skin tending to resist the passage of the electric current and thereby increasing the electrolytic tension of the cell. Moreover this resulting increase in tension tends to bring about still other obstacles to the action taking place at the anode, due partly to local elevations of temperature, and partly to a tendency to increase the electrolytic potential at the surface of the anode. The harm of these disadvantageous actions is that in due time the increasing potential will reach that potential at which the iron impurity starts to dissolve at the external layers of the anode.

\Ve have determined however that the described interfering alteration of electrolytic tension at the surface layers formed by the undissolved impurities of the anode is materially smaller at the low current densities than at high ones. In other words there is a critical potential at which the iron and other undesirable impurities commence to go 100 I anode,

on the time,

9 sions (length and breadth) into the solution, which critical point is not reached when usin the low current densities, or is only reach when the aluminum has been dissolved from the deeper layers of the whereas with the hig er current densities a reciable amounts of iron and silicon inevita y pass into solution.

While it therefore appears that'the lower current densities are desirable in order to 10 reduce the amount of impurity in the product deposited at the cathode, yet, on the other hand, it is considered to be necessary, in a commercial sense, to operate with current densities as high as possible in order to carry rocess in t e most economical way to afior the maximum output. The reason for this latter consideration is that the hi h current densities produce at the oath es moremetal per unit of surfacein a given thereby not only increasing rate of output, but effecting an economy in electrolyte material and a saving space and cost. For instance, production economies may be effected by reducing the size of the electrodes and thereb the quantity of electrolyte, which at the same time means a substantial saving of current employed in the electric heating of the electrol te.

Clearly t erefore it would not be economical to employ a relatively small size of cathode in conjunction with a large size of anode inorder to permit high current densities at the cathodes and low densities at the anodes, because by this plan it would not be possible to reduce the size of the cell, nor the quantity of electrol te nor the amount of current required. either are the desirable results obtainable-by the use of cathodes and .anodes which have the same size or dimenand the usual platelike form with plane sides, because with this plan it would not be possible simultaneously to maintain high current densities at the cathodes and low densities at the anodes. In order to insure a thorough exhaustion of the anode material, anodes in the form of a plain plate would have to be made relatively thin, and would therefore have to be replaced with objectionable frequency, or

9 else the total amount of electric current assed through the bath would have to be resuced considerably, consequently rendering far less the rate of metal produced per unit of surface, resulting in a most uneconomical system.

According to the present invention the difliculties referred to are altogether avoided and the desired advanta es are obtained by the employment, in the fescribed process, of anodes each having a substantially larger surface than an anode of the same size or dimensions and the same weight, but with plain l or flat sides or surfaces. The present invention is characterized by presenting the solid anode metal in a form so departing from a creased, not merel It is not at first sight clear that such anodes,

formed with elevations and depressions sub-' stantially enlarging the surface of exposure, can be used in the described process, because of the known fact that under ordinary conditions the lines of current flow prefer prominent surface portions or elevations to flat portions orde ressions and therefore tend to be far more ense at angles, jections than at flat surfaces, depressions or ooves. From this consideration it might e expected that the disintegration of the special anodes contemplated by this invention would tend to take place only at the outedges and prostanding portions or elevations and not at the flat portions or depressions, which, if true, would render practically impossible a systematic or complete exhaustion of the anode material.

We have determined however by test and demonstration that with the present invention this drawback does not occur, and that, notwithstanding the expected tendency, the distribution of the current remains practically uniform over all of the surface of the anode. We attribute this largely to the character of electrolyte employed, which has very good conductivity. Actually therefore, the special anodes of this invention, with their large exposed surface, are decomposed in a substantially homogeneous manner. The invention thus makes ossible a far better utilization of the anod material than is obtainable with the use of anodes havingthe same size or dimensions and weight, but having lain side surfaces thlc 'er in section.

Anodes, according to this invention, may be of various form or design, and may afford a surface from twice to eight times the extent of an anode havin the same length and breadth but with flat sides. Althou h in using a simple plate or slab as an ano e the tension of the electric current tends to exceed the admissible limit or critical point described as soon as thirty per cent more or less of the contained aluminum has been dissolved, we find that an anode of the same length. breadth and weight, but whose surface has beenmade. I large according to the present invention, may

and being accordingly 113,561 and containing electrodes and electrolyte operating according to the present invention.

The cell 5 is shown as having an entering pipe 6 to permit the circulation or replacement of electrolyte, and an exit pipe 7. The electrolyte or bath 8 is a molten bath having a melting point lower than and maintained below the melting point of either electrode. As described in the prior applications such bath may contain halogen-salts, namely halides of aluminum and halides of another metal or metals having alkaline properties,

and the bath having good electric conductivity. A typical bath comprises halogen salts of aluminum together with halogen salts either of one or more alkali metals or one or more alkali earth metals, or all of these salts,

as more particularly disclosed in the prior applications. A simple bath consists of NaCLAlCl- In the bath are shown immersed alternated catholes 9 in the form of plain slabs or plates and anodes 10 of s ecial form, but of the same size, that is lengt and breadth, as the cathodes.

The special anode 10 is shown as formed to present enlarged exposed surface by means of a system of elevations or ribs 11 and de pressions or grooves 12. This undulated form materially increases the surface. Preferably the ribs and rooves are formed at both sides of the an e and in such manner that the ribs at one side are 0 posite to the grooves at the other side, so that the anode is given a substantially uniform thickness from surface to surface at every oint, and presents no parts which, being thlcker than others, would still contain aluminum, while the thinner parts of the anode would already be completely exhausted. An analogous increase of exposed surface may be effected in various other ways, for example by having the ribs and grooves run in intersecting directions so as to form prominences of the shape of pyramids. The anode may be produced not only by casting but b rolling or by extrusion into appropriate s apes.

The electrical connections may be made in various ways, but for example, as shown, the

several anode plates 10 and the cathode plates 9 are connected in parallel. Thus a conductor 13 connects the cathodes and a conductor 14 connects the anodes, and the roper current may be supplied to these con uctors both for the purpose of heatin the electrolyte to maintain it in molten con ition, and for electrode osition. Or the heating may be effecte by a secondary current or by external means. Other ways of electrical connections may be applied, for example the connection in series, es ecially according to the well known Hay en or series system, which has been known and used for many years, for example in copper refining, and which is described in. various publications, such as Enineering and Mining Journal, New York,

01. LIV (1892) page 126. p

. An illustrative instance, within the principles described, the anodes and cathodes may be of such relative character or arranged that a cell voltage of 0.3 volt between anodes and cathodes the current density at the anodes may be 0.5 amperes or less per square decimeter, and at the cathodes 1.0 amperes or more, with an appropriate distance of sep aration and electrolyte of'high conductivity as described.

There has thus been described a process for producing highly pure metallic aluminum from less pure metal and a'special electrode or anode for use therein'embodying the. principles and attaining the objects of the present invention. Since various matters of process, ingredient and apparatusmay be variously modified without departing from the principles of the invention, it is not intended. to limit the invention to such matters except so far as set forth" in the appended claims.

What is claimed is:

1. A process of electrolytic extraction of pure aluminum by therefining of pig alumi num, aluminum alloys or the like, and wherein both the aluminum containing anodes made of the material to be refined and the cathodes are used in a solid form together with an electrolytic bath having a melting point lower than that of either electrode, and consisting of halogen salts of aluminum and of alkaline and/or alkaline earth metals, characterized by the use of an anode having a substantially larger surface than an anode of the same size and the same weight but with plane surfaces.

2. The electrolytic process of producing highly pure metallic aluminum from less pure metal wherein the electrodes in solid form are immersed in a molten electrolyte containing halogen salts and having a melting point lower than and maintained below the melting point of either electrode and characterized by presenting the solid anode metal in a. form substantially departing from a plain slab or plate so as to afford substantially enlarged surface of exposure, as compared with that of the cathode.

3. The electrolytic process of producing highly pure metallic aluminum from less pure metal wherein is used a molten electrolyte having a melting point lower than that of the anode, and characterized by the use of a solid anode in a form departing substan- 5 tially from a plain slab or p ate so as to present extensive surface of exposure.

4. The electrolytic process of producing highly pure metallic aluminum from less pure metal wherein the electrodes in solid form are immersed in a molten electrolyte possessing good conductivity and having a melting point lower than and maintained below the melting points of the electrodes, and characterized by presenting the impure metal as solid anodes having substantial elevations and depressions presenting extended surface exposure.

5. The combination with a molten electrolytic bath and means for supplying current,

90 a series of cathode plates and alternated therewith a series of solid anodes formed with substantial surface elevations and depressions, so as to present a larger surface than that of the cathode, whereby to causea larger current density at the cathode than at the anode.

6. The electrolytic process of producing highly pure metallic aluminum from less pure metal wherein the electrodes in solid form are immersed in a molten electrol te containing halogen salts and havingameltmg point lower than and maintained below the melting point of either electrode and where in the anodes and cathodes are of such difi'erent character or arrangement that the current density at the anodes isv substantially less than at the cathodes.

In testimony whereof, this specification has been duly signed by:

JULIUS WEBER.

ALFRED VON ZEERLEDER.

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